An image heating device (fixing device) for heating and fixing a toner image transferred onto a sheet of paper is disclosed in Japanese Patent Application Publication No. 2001-125407.
The fixing device described in Japanese Patent Application Publication No. 2001-125407 includes a heat roller having a high permeability in which the Curie temperature is set a predetermined value, a pressing roller for making contact with the heat roller, thereby forming a nip, an exciting coil for exciting the heat roller from the outside, an exciting circuit for driving the exciting coil, and a conductive material with a semicircular section which is arranged inside the heat roller, has a higher conductivity than that of the heat roller (that is, the electric resistance is low), and can be rotated. When the temperature of the heat roller approaches the Curie temperature, and the permeability is lowered, and the conductive material is rotated to the opposite position to the exciting coil, the magnetic flux passing through the heat roller approaching the Curie temperature penetrates into the internal conductive material. Here, by controlling the current to be supplied to the exciting coil constant using the exciting circuit, generation of heat is suppressed and the temperature of the heat roller is made stable.
However, in the above constitution, the exciting circuit controls the current to be supplied constant, though the heat roller and the electric resistance of the conductive material depend on the temperature, so that the power supplied by the exciting circuit is not controlled. Recently, there is an increasing demand for controlling appropriately the distribution of the supply power of the whole image forming apparatus, and if the power control is made unconditional, a problem may be caused. On the other hand, when intending to control the power constant, if the permeability of the heat roller is lowered, the effective magnetic flux does not stay in the heat roller and passes through it, thus the impedance of the entire exciting circuit is lowered, so that there is a fear that the current flowing through the exciting circuit may exceed the allowable current.